User:Gisselle Medina/Sandbox1 Carbonic Anhydrase



Abstract
Respiratory control is a physiological function believed to be dependent on intracellular and extracellular pH concentrations as well as CO2 chemoreception. In a cell membrane, there are many factors that control intracellular acidity, including carbonic anhydrase, a sodium-hydrogen exchanger (NHE), and an anion exchanger. By modifying the activity of any of these factors, the mechanism by which CO2 reception occurs can be identified. Inhibition of carbonic anhydrase by acetazolamide in mouse and cat slice preparations has provided one strategy to see how CO2 chemoreception works. If this enzyme is inhibited, it is believed that the reaction CO2 + H2O  HCO3- + H+ will slow down due to losing its catalyst, causing a buildup of CO2 and a slower rate of H+ ion formation. With no rapid or significant decrease in pH, chemoreception of CO2 will neither occur nor produce any considerable changes in the frequency of breathing. However, researchers have found that even when CA is inhibited, and the cell is placed in a hypercapnic environment, increased frequency of breathing still occurs. Increased arterial concentration of CO2 drives the reaction to the right, thus increasing pH to produce tissue acidosis and increasing ventilation and breath frequency. Therefore, carbonic anhydrase does not appear to be essential in CO2 chemoreception. This conclusion is still an important discovery, however, as it demonstrates the importance of inhibitor-enzyme models in focusing researchers’ interests in the right directions.

Our Model
Carbonic anhydrase XIII, which is found in the brain and many other tissue types, is modeled here with an acetazolamide inhibitor (green). The sulfonamide nitrogen atom of the inhibitor replaces the water molecule that would have formed a complex with the enzyme’s catalytic zinc atom to hydrate the CO2 molecule. The zinc-acetazolamide complex is further stabilized by hydrogen bonds with the coordinating imidazolic nitrogen atoms of His94, His96, and His 119 (blue). Acetazolamide is extremely effective not only because it binds well to the zinc complex, but also because it is involved in many other interactions outside of the catalytic site. An oxygen of the sulfonamide group hydrogen bonds with the nitrogen-hydrogen backbone of Thr 199. The thiadiazole ring forms strong van der Waals interactions with residues Gln92, Val121, Leu198, Thr199, and Val200 (red), while the acetomido group establishes the same interactions with Phe131 (orange). The unique interactions between carbonic anhydrase and acetazolamide allow for more detailed analyses to be done concerning the activities of cells that are CO2 chemosensitive and play an important role in respiratory control.